The present invention relates to automatic watering systems for batteries and fuel cells, and, more particularly, to a combined gas purging and watering system for vented secondary batteries and fuel cells, and the watering caps used therein.
For various reasons, all secondary cells and fuel cells utilizing an aqueous electrolyte such as, for example, cells containing lead and lead oxide electrodes in a sulfuric acid electrolyte tend to undergo changes in volume of electrolyte during charge and discharge. For example, electrolyte loss often occurs during the charging process due to electrolysis (decomposition) of water in the electrolyte into its constituent elements, hydrogen and oxygen.
The gases generated by electrolysis during charging, when undiluted, form an explosive mixture. However, it is not desirable to completely eliminate such generation of gas. During the charging period, sulfuric acid is generated and tends to have an increased concentration adjacent to the portions of the electroplates where the majority of the electrochemical reaction occurs. Stratification of acid concentration can reduce the available discharge rate of the battery. A low level of gassing, providing slowly moving gas bubbles through the electrolyte, serves to distribute the generated sulfuric acid.
Loss of electrolyte also occurs due to evaporation, particularly in view of sustained periods of heat generation typically occurring during charging and high rate discharging of the battery, which often occur when a battery is used to power an electric vehicle. Such evaporation is particularly manifest in cells near the center of a large battery pack, due to heat entrapment.
If the level of electrolyte in a given battery cell drops below the tops of the electrode plates, irreparable damage to the plates can occur. Increased corrosion and shedding of active material tend to result, to the detriment of cell life and capacity. Moreover, there is an increased risk of arcing between exposed, and possibly faulty, plates, which could ignite any inflammable gases present. Conversely, the onset of higher temperatures often causes an initial expansion of the electrolyte in the cells. It is therefore necessary not only to allow for expansion of the electrolyte in the battery cells, and to vent and disperse explosive gas mixtures, but also to periodically add electrolyte to the cells to compensate for losses.
In general, automatic watering systems for replenishing electrolyte and purging gases are known. However, the prior art systems are disadvantageous in that they require special battery configurations, or require separate exhausting and electrolyte replenishing systems, or both.
Many such systems utilize battery covers which greatly restrict access to the contents of the cells. Other systems involve moving parts which are liable to malfunction.
Also, prior art watering systems often utilize a pump which is powered by an external source of power, typically line current. Thus, the watering systems can operate only when the vehicle is not being driven. In addition, such systems are typically manually operated, requiring an operator to deactivate the pump after pumping sufficient water to replenish the cells.
The removal of the explosive gases from the vicinity of the batteries in electric vehicles is usually effected by fans blowing air over the tops of the cells. Hydrogen is thus diluted to a concentration below 4%, (the recognized lower limit for spontaneously explosive mixtures of hydrogen and oxygen). However, pockets of explosive gas mixtures in the vicinity of the battery are sometimes difficult to detect and disperse. To this end, flame traps are often incorporated into automatic watering systems to prevent explosion or combustion from being propagated along pipes connecting watering caps when the vehicle is being driven. Examples of prior art systems are described in U.S. Pat. Nos. 1,302,648 issued on May 6, 1919 to Flanders; 3,434,887 issued on Mar. 25, 1969 to A. E. Seckinger; 3,664,876 issued to Carl on May 23, 1972; 4,087,592 issued May 2, 1978 to Okazaki et al; and 4,286,027 issued Aug. 25, 1981 to Shropshire et al. Another such system is described in Offenlegunschrift 2,303,244 by Tanaka et al, published July 25, 1974.